The present invention relates to a marine propeller and, more particularly, to a propeller design which is capable of efficient adaptation for a wide range of propeller speeds and cavitation conditions.
The operation of propellers is based upon principles which have been generally accepted for many years, wherein, for example, effective performance depends upon maintaining an optimum pressure differential between the opposite surfaces of the propeller blades. However, application of basic propeller principles to actual operating and design conditions involves the interplay of numerous variables such as the blade speed, the multidirectional fluid flow, the vapor pressure of the surrounding fluid, induction of air into the fluid, the configuration of the surfaces of the propeller blades, and cavitation on and adjacent to the surfaces of the propeller blades. Thus, design of an efficient marine propeller, which would seem at first glance to be rather simple, actually involves many complex factors and requires much trial and error. Also, slight changes and modifications of a proposed design may have great and unobvious effects on the operating characteristics and efficiency of the propeller.
Cavitation represents an important consideration with respect to efficient operation of the propeller, reduces noise propagation and improved blade wear. Cavitation results from a local reduction in fluid pressure to a value lower than or equal to the vapor pressure of water. A cavitating flow is naturally produced in a liquid when the relative speed between an immersed body and the liquid is great enough to induce sufficiently low fluid pressures so that the liquid is caused to vaporize and produce a cavity. A cavitating flow may also be produced through the artificial introduction of a gas, such as air, into a liquid in the vicinity of the immersed body so that a cavity is more easily formed thereabout. Each cavity usually contains a swirling mass of droplets and vapor which suddenly collapse when the surrounding fluid pressure becomes high enough and the collapsing fluid, which rushes to fill the void, momentarily raises the localized fluid pressure to a high peak value. If the point of collapse is in contact with a propeller blade, the surface of the blade receives a blow which may erode such surface; cause a large amount of vibration and noise; and reduce the thrust efficiency of the propeller.
Cavitation on the propeller blade can be avoided by limiting the propeller speed and using larger propeller blades. However, this solution soon reaches practical limits inasmuch as the bearings, gears and power train elements soon take up a large amount of space that could be used for other purposes. Another means of avoiding cavitation damage on the surface of a propeller blade is by utilizing a supercavitating blade which causes a cavity of sufficient length so that it extends downstream of the blade and has its point of collapse also downstream of the blade. However, supercavitating propeller blades are not very efficient at low speed, noncavitating conditions since the pressure distribution and the lift to drag ratios of the propeller blade are distinctly different than those parameters for high speed cavitating conditions. At such noncavitating or subcavitating conditions propeller blades are commonly used which have rounded leading edges and tapered trailing edges, as distinguished from supercavitating blades which normally have tapered leading edges and blunt trailing edges.
For particular applications such as with planing type boats, which employ high speed propellers positioned close to the water surface, or medium speed hydrofoils where both subcavitating and supercavitating conditions occur adjacent the propeller blades, there has not been generally available a propeller blade design which is capable of efficient operation, reduced noise generation and improved wearability. In intermediate flow states with co-existing supercavitating and subcavitating flow conditions, subcavitating propellers tend to induce oscillating cavitation cavities and supercavitating propellers are highly inefficient.